Australian officials say the extreme fire danger has not passed for the country's southeast despite an easing in searing temperatures.

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It has been a tense night for firefighters as they worked to control hundreds of fires across New South Wales (NSW), ACT and Victoria.

A cool change early this morning saw the mercury plunge 10 degrees over two hours in Sydney, bringing to a close one of the worst ever fire days with only a handful of homes have been destroyed and no lives lost.

But the reprieve would be short-lived, with authorities tracking a dangerous band of hot air moving north toward Queensland and temperatures expected to climb again by the end of the week.

Rural Fire Service Commissioner Shane Fitzsimmons said the southerly was bringing relief from the heat and humidity, but was not helping fire crews.

Fresh fires were still breaking out, including one in the Blue Mountains in Lithgow, where flames were visible from the Great Western Highway.

''We've got reports of new fires occurring in places like Lithgow in the central ranges, west of Sydney,'' Fitzsimmons said.

''And of course we've got that very hot air moving northward across much of the northeastern areas of NSW today, which is why we have got statewide total fire bans in place today.''

He said they couldn't afford to relax.

The RFS said 1700 firefighters were battling 140 fires across NSW alone - 30 of them were out of control.

Hundreds of fires closed in on tinder-dry rural settlements and picturesque coastal holiday spots throughout New South Wales yesterday.

The high temperatures and furnace-like winds, often topping100kmh, combined to produce a perfect storm for Australia's bushfire-prone eastern seaboard, escalating fears that NSW would be subjected to the devastation experienced in Tasmania last week.

While NSW and Tasmania were the focal points of an anxious 24 hours, resources were also stretched in Victoria and the ACT, while firefighters also had blazes to put out near Alice Springs and in southern Queensland.

Homes near Ballarat in Victoria were destroyed and seven people treated in hospital after an out-of-control grass fire swept through Snake Valley.

Authorities rushed to contain the blaze - one of at least 13 fires burning in the state on Tuesday night - which grew quickly to about 1100 hectares.

Four properties were believed to have been lost to fire. Historical homestead Carngham Station, a grandiose two-storey brick villa set on more than 2000 hectares with a 30-hectare lake was destroyed.

A cool change was expected to bring relief for Tasmania, where fires were still burning and more than 100 homes were razed to the ground.

The state has continued to provide the most harrowing images - and survival stories - associated with "catastrophic" bushfire conditions.

As a group of 13 New Zealand volunteer fire fighters headed for the Tasmanian capital Hobart to relieve exhausted locals, some of Boomer Bay's 262 residents returned to the tiny coastal settlement and sift through the remnants of their fire-ravaged properties.

Situated north of Dunalley on the Tasman peninsula, half of the hamlet's buildings were destroyed while the carcasses of burnt-out vehicles dotted along the waterfront to resemble a war zone.

In scenes mirroring the panic that spread through Dunalley at the weekend, 50 Boomer Bay residents took refuge on a 15-metre long wooden jetty as flames licked the beach.